1. Field of the Invention
This invention deals generally with neural prosthesis, specifically the concept of achieving a retinal prosthesis for blind patients through the creation of an electrical interface between a high-density electrode array and the curved surface of the retina.
2. Description of the Related Prior Art
There is a great deal of recent interest in the area of neural prosthesis, specifically the concept of achieving a retinal prosthesis for blind patients has been hypothesized by a number of researchers and is an active area of medical research. In a normal eye, in a basic concept 10, FIGS. 1a and 1b shows a ray trace of two photons 12 focused on a retina 21. Note that the incoming photons 12 pass through several layers of transparent retinal cells 16 and 18 before being absorbed by the photoreceptors 22. In a damaged eye, a retinal prosthesis device 24, as shown in FIGS. 1c and 1d, is positioned against the retina 21. In this case, the photons 12 are absorbed by a microelectronic stimulating array or device 26 that is hybridized to a glass piece 28 containing an embedded array of microwires. The glass has a curved surface that conforms to the inner radius of the retina 21. The microelectronic imaging device 26 is made of thin silicon containing very large scale integrated (VSLI) circuitry and photon detectors that convert the incident photons 12 to an electronic charge. The charge is then converted to a proportional amount of electronic current which is input to the nearby retinal cell layer 18. The cells fire and a signal is sent to the optic nerve 28.
A typical retinal prosthesis device combines two technologies: first, nanochannel glass (NGC) electrode arrays and secondly a two-dimensional (2-D) multiplexer array. NGC technology employs fiber optic fabrication techniques to produce thin wafers of glass with very small channels perpendicular to the plane of the wafer. Typical NGC wafers that will be required for retinal prosthesis devices are several millimeters in diameter and can contain millions of channels with channel diameters on the order of one micron. The channels are filled with a good electrical conductor and one surface of the glass is ground to a spherical shape consistent with the radius of curvature of the inside of the retina. The electrical conductors on the curved surface should protrude slightly to form efficient electrodes.
The 2-D multiplexer array is similar to infrared focal plane array (IRFPA) multiplexers that are microelectronic devices fabricated at silicon foundries. An IRFPA multiplexer is a 2-D array that reads out the infrared (IR) image captured by a complimentary detector array that converts photons into electrical charge. The charge is integrated and stored in each unit cell for a few milliseconds. The full image is then multiplexed off the array at frame rates compatible with commercial video. For a retinal prosthesis test device that obtains its input image from an external camera, the process is essentially reversed and the device acts as a de-multiplexer. That is, the prosthesis devices will perform de-multiplexing operations, but will be referred to here simply as a multiplexer.
The basic concept is straightforward: visual images can be produced in the brain by electrical stimulation of retinal cells. Two-dimensional arrays of retinal cells, such as ganglion or bipolar cells, can be stimulated using two-dimensional arrays of electrical impulses with the spatial form of an image. The axons of the ganglion cells then transmit the image through the optic nerve and on to the visual cortex. This is in lieu of the normal photo-transduction process that occurs in a healthy retina. In approximately 90 percent of blind patients, the photoreceptors are diseased, but the other retinal layers are still responsive to electrical stimulation.
Experimental test procedures, such as shown in FIG. 2, use standard retinal surgical techniques performed in an operating room environment by an ophtalmologist. It is necessary that the patient be administered local anesthesia rather than general anesthesia so that visual perceptions can be orally recorded during the procedure.
There are a number of technical issues to be addressed in designing and fabricating a retinal prosthesis device, particularly if the device is to generate a high resolution image. First, there is the issue of creating an electrical interface between the high-density electrode array and the curved surface of the retina. The electrode array must have a spherical, convexed shape in order to conform to the spherical concaved surface of the retina. The electrode array must be bio-compatible and safe for permanent implantation. Second, the electrical stimulation pulse shapes and repetition rates, while generally well known, may need to be optimized for each individual recipient of a prosthesis device. The pulse amplitude is of course modulated within the retina to be proportional to the pixel value. Third, direct electrical stimulation of the ganglion cells precludes certain image processing functions that normally would have occurred in earlier layers of the retina. Therefore, computationally based image preprocessing operations may need to be performed on the image before stimulation of the retina. Fourth, supplying power to a permanent implant will need to be engineered in a manner such that there are no wires or cables through the eye wall. Fifth, because a normal retina processes image information created by the photoreceptors in a simultaneous manner, it is assumed that a prosthesis device should similarly excite retinal cells in a simultaneous manner, as opposed to sequential raster scan that might cause synchronicity problems downstream in the lateral geniculate nucleus (LGN) or visual cortex.